Plugins - Generic plugins framework

 use Plugins;

 $plugins = Plugins->new context => $context;
 $plugins->readconfig($config_file, self => $self);
 $plugins->initialize();

 $plugins->invoke($method, @args);

 $plugins->invoke_until($method, sub { scalar(@_) }, @args);

 my $iterator = $plugins->iterator();
 while (@results = &$iterator(@args)) {
 }

 for my $plugin ($plugins->plugins()) {
	$plugin->invoke($method);
 }

Plugins provides a simple way for a programs to be assembled at runtime via configuration files.

Generally, there are only a few ways that a module in a library can be customized for an application: (1) You can change the code of the module; (2) if it presents an object-oriented interface you can subclass it; or (3) the module in question can have a configuration file that allows you to do what you need. Plugins makes it easier to allow behavior to be modified by a configuration file by allowing other code to be mixed in.

Plugins allows plugins have plugins and for all the plugins to share a single configuration file. This isn't required but sometimes it's nice to have everything in one place.

Each plugin is implemented by a perl module.

There can be more than one instance (object) for each plugin (class).

The Plugins module itself isn't complete and must be subclassed. An example subclass that completes Plugins is Plugins::Style1. Plugins written for use with the Plugins module generally don't need to know how Plugins has been subclassed because the $context that is passed to a plugin allows it to use Plugins directly even if its ancestor did not.

Since plugins can have plugins, some plugins will have Plugins objects themselves. Some won't. We will term a user of Plugins to be requestor. A requestor that is not itself a plugin will be termed the root. A plugin will be termed a child and the requestor that invoked it will be termed the parent. A plugin that has plugins is a child requestor.

While there are similarities, using Plugins for the first time (by the root) is different than using it as a plugin that has plugins (child requestor).

The root requestor will need to create a plugins object. Since Plugins needs to be subclassed it will create the object using the subclass. For example, Plugins::Style1:

 $plugins = Plugins::Style1->new(%args);
 $plugins->readconfig($configfile, %args);
 $plugins->initialize();

There are three steps for a requestor to take to start using plugins. Each step is a call to Plugins:

 $plugins = Plugins->new(context => $context, %args);
 $plugins->readconfig($configfile, %args);
 $plugins->initialize();

The most important argument for Plugins::new() is context = $context>. The $context comes from the first argument to child->new() when Plugins creates plugin object instances.

For child requestors, no %args are needed for new() except for context = $context>.

Likewise, no %args are needed for readconfig().

There may be additional parameters depending on how Plugins is subclassed.

The %args that new() userstands are:

configfile => "/config/file"

Provide a configuration file name. If this is done here then undef can be passwd for the configfile to readconfig().

api => $api

Provide a Plugins::API object that will be passed to any plugins's new() invocation.

Readconfig has a positional argument: $configfile:

 $plugins->readconfig($configfile, %args);

%args for readconfig() depend upon how Plugins is subclassed.

Plugins provides the following methods for requestors:

invoke($method, @args)

This calls each plugin in turn. Plugins are called in the order in which they were configured. No return value is defined. The calls are not eval-wrapped so a die in a plugin is fatal.

invoke_until($method, $testfunc, @args)

This calls each plugin in turn. Plugins are called in the order in which they were configured. After each call, the return value from the plugin method invocation is evaluated with &$testfunc(). If &$testfunc() returns a true value, then the return value from the plugin method invocation is returned and no further calls are made.

plugins()

This returns the list of plugin objects. This will be in the order in which the plugins were defined.

iterator($method)

This returns an anonymous function that when invoked will call the first plugin ($plugin-method(@args)). Each successive call will call another plugin until it returns undef.

startconfig()

This method can be used instead of readconfig() to start the configuration process. It does not read any configuration files. It is required prior to parseconfig() or initialize().

parseconfig($configfile, %args)

This parses a configuration file. Unlike readconfig() it does not call startconfig() first.

api($api)

This will set an $api variable that will be passed to new() when creating new plugin instances. This is expected to be a Plugins::API object.

The readconfig() method may be called more than once. Each time it is called, initialize() may be called again. When initialize() is called a second time, it calls $plugin->shutdown() for each of the old plugins before calling $plugin->new() to create the new ones.

Each call to readconfig() starts fresh. If you want to add to an existing set of plugins rather than replace them, use parseconfig() instead of readconfig().

Plugins can be either a perl module that can be found by looking in @INC or they can be files that are wrapped and eval'ed by the Plugins module.

For plugins that are files, they will be wrapped with a bit of code:

	package Plugins::AutoGenerated::A_UNIQUE_GENERATED_VALUE;
	our @ISA = qw(Plugins::Plugin);
	use strict;
	THE_CONTENTS_OF_THE_FILE

This wrapping is done by file_plugin() method.

For plugins that are regular perl modules and thus not auto-wrapped, they should declare themselves to be subclasses of Plugins::Plugin.

Plugin objects provide the following methods:

invoke($method, @args)

Basically this just does $plugin->method(@args). If $method does not exist, just return undef.

new()

The default new() does the following:

 sub new
 {
	my ($pkg, $pconfig, %args) = @_;
	return bless { 
		context	=> $pconfig->{context}, 
		api	=> $pconfig->{api}, 
		config	=> \%args 
	}, $pkg;
 }

You'll often want to override new().

Plugins will need to define methods to be called. What methods need to be defined depends upon what they are a plugin for. Plugins for programs that are a Daemon::Generic will generally need preconfig() and postconfig() methods. Look at the documentation or code of the requestor.

The following methods are called directly by Plugins:

new($pconfig, @args)

Invoked to create an object instance of a plugin. The @args come from the configuration file. The $pconfig is used to pass in plugin-related configuration data. It's a reference to a hash and $pconfig->{context} needs to be passed to any child requestors so that they can fit themselves in properly.

shutdown()

Called when a object instance is no longer needed due to a reconfig after the configuration file has been changed. On a reconfiguration, all the old objects are destoryed and a new set are created. The default shutdown() doesn't do anything.

The Plugins module has defines an AUTOLOAD function in Plugins::Plugin base class to map unknown method invocations into invoke(). It does this through the Plugins::API module. Plugins that invoke a method that isn't formally defined will have the attempted method invocation redirected through $self->{myapi}->invoke() or $self->{api}->invoke().

The default new() for plugins will capture a Plugins::API object passed in from Plugins as $self->{api}. Plugins that have plugins might want to name their Plugins::API object $self->{myapi} (assuming they have one).

Plugins that themselves have plugins (child requestors) will need to invoke Plugins themselves. If the overall program uses Daemon::Generic, then doing this in a preconfig() method is reccomended. In other contexts this may need to be done in new().

 sub new
 {
	my ($pkg, $pconfig, %args) = @_;
	return bless { 
		context	=> $pconfig->{context}, 
		api	=> $pconfig->{api}, 
		config	=> \%args 
	}, $pkg;
 }

 sub preconfig
 {
	my ($self, $configfile) = @_;
	my $config = $self->{config}{configfile} || $configfile;
	$self->{plugins} = new Plugins %{$self->{context};
	$self->{plugins}->readconfig($config, self => $self);
	$self->{plugins}->initialize();
	$self->{plugins}->invoke('preconfig', $config);
 }

It is expected that whichever subclass of Plugins is used by the parent should also be used by the child. The following snippet will supress this behavior. This is probably a bad idea unless the child uses a different configuration file than then parent.

 local($self->{context}{pkg_override}{$config});
 $self->{plugins} = new Plugins context => $self->{context};
 $self->{plugins}->readconfig($config, self => $self);

If you don't want to use an existing configuration file format, you don't have to.

Subclass Plugins and override a couple of methods to change the behavior.

In addition to the methods defined above in the USING PLUGINS and in CONSTRUCTION AND INITIALIAZATION that probably shoudn't be changed, Plugins defines the following methods:

parseconfig($configfile, %args)

This is the method that actually parses a config file. This must be overridden as the default just calls die().

The parameters come from either a direct call from the requestor or are passed through unchanged from readconfig().

post_initialize($context, $plugin)

Called by initialize_plugin() after new'ing up an instance. The default behavior is to do nothing. The $context is the same context that is passwed to new() and pkg_invoke() and addplugin().

genkey($context)

This is called to generate a unique identifier for each plugin instance. Duplicate identifiers will result in a die(). The default genkey() combines the configuration file name with the plugin package name and thus does not allow multiple instances of the same package.

$pkg = file_plugin($file, %opts)

This will look for $file and then auto-wrap it with a package declaration to turn into a plugin perl module. The auto-generated package name is returned.

%opts can be:

search_path

If $file isn't an absolute path, search for it in @{$opts{search_path}}. No search is done if unspecified.

referenced

Where was $file referenced from? (eg: "/some/config-file, line 33") Used in error messages only.

prefile

Code to insert in the eval just before the file's contents.

postfile

Code to insert in the eval just after the file's contents.

isa

The value for the generated plugin's @ISA. Defaults to Plugins::Plugin.

registerplugin(%context)

This add a plugin to a configuration. This is used after startconfig() and before initialize(). This will require the plugin unless the symbol table for the plugin already exists. This method should not be overridden.

addplugin(%context)

This is used to bypass the entire startconfig(), registerplugin(), invoke() process. The plugin will be registered and initialized all at once. This can be used after initialize() to add additional plugins. This will require the plugin unless the symbol table for the plugin already exists. Duplicate registrations of the same plugin will replace the old plugin (shutdown() will be called). This method should not be overridden.

The %context is the same as for %registerplugin()

pkg_invoke($pkg, [$method, @args])

This method will require a plugin and (optionally) call a class method. This method should not be overridden.

initialize_plugin($self, $context)

This is used by addplugin() and initialize() to create plugin instances. It calls new() and then post_initialize(). This method should not be overridden.

Most of the %context has that is given to addplugin() and registerplugin() should (in theory) be passed as $context to the plugin's new($context, %args) and then as context => $context to Plugins::new() if the is a child requestor.

The part that is not passed to Plugins::new() is the part that defines the child plugin:

pkg => 'Some::Plugin::Module'

The perl package name for the plugin.

new_args => [@args]

The @args will be passed to the the plugin's new():

 new $pkg (\%context, @args)

requestor => 'The::Parent::Module'

The perl package name of the parent module. This will be found with caller() if it isn't supplied.

configfile => "/some/file"

Which configuration file referenced the plugin and caused it to be loaded.

lineno => 38

The of where the plugin was registered in $configfile. Used for error messages.

The rest will be passed to child Plugins::new() by way of $context:

pkg_override => "Plugins::Subclass"

Plugins has been subclassed, this the name of the subclass. Subclassing should usually be inherited by child requestors and this is the mechenism that makes it happen.

The tricky part of subclassing Plugins is that if a configruation file is shared between root requestors and child requestors the parseconfig() method will be invoked on the same file more than once.

Store extra things in %context to work around this issue.

Plugins::Style1 Plugins::API

If you find this module useful and wish to show your appreciation to the author, please give me a Request-For-Quote on your next high-speed internet pipe order. I have good pricing for T1s, T3s, OC3s etc.

Copyright (C) 2006, David Muir Sharnoff <muir@idiom.com>. This module may be used and redistributed on the same terms as Perl itself.